The present invention relates to video modulator arrangements by means of which audio-frequency and video signals may be modulated onto a high frequency carrier signal. In particular although not exclusively the invention is concerned with multi-standard video modulator arrangements.
Video modulator arrangements up-convert baseband signals, namely audio and video signals, to VHF or UHF carrier frequencies in the range from, say, 40 MHz to 900 MHz. International standards are set down for the modulation parameters in the various systems, PAL, NTSC and SECAM, but in general the audio-frequency signals are first modulated onto a subcarrier, and the modulated subcarrier frequency components are combined with the video signal components for modulation onto the VHF or UHF carrier signal.
In SECAM systems, where the audio signal subcarrier is amplitude modulated at up to 80% modulation depth, known forms of modulator arrangement have given rise to unwanted variations in main carrier amplitude, which produce an effect known as sound in vision.